Some of traditional engines employ a valve overlap period in which respective open states of an intake valve and an exhaust valve overlap with each other for the sake of improving scavenging efficiency of combustion gas and/or charging efficiency of intake air. An engine equipped with a turbocharger, for example, can improve the charging efficiency by prolonging the valve overlap period, achieving a rise in engine output. In view of this, techniques have been developed to make opening/closing timings of the intake valve and/or the exhaust valve of the engine variable and to control the valve overlap period in response to an engine load and the like (e.g., Japanese Patent Laid-Open No. H7-11962).
However, in an engine equipped with a port injector, the longer the valve overlap period is set, the easier it becomes to cause so-called pass-through, which is a phenomenon that the mixture flowing from the intake passage into the cylinder passes directly through to the exhaust passage. When the pass-through occurs, the fuel supplied by port injection passes through the cylinder and flows out toward the exhaust passage, so that engine output and/or exhaust performance may be deteriorated. This pass-through is more likely to happen when the target torque of the engine is large (i.e., as the load increases) and tends to occur, for example, as supercharging pressure becomes higher than the exhaust pressure. In addition, during travels in high altitudes whose atmospheric pressure is lower than the normal atmospheric pressure, the density of air is decreased, so that, if the valve overlap period is set as with the case in the normal atmospheric pressure, the output may drop due to the charging efficiency not reaching the desired value.
The present disclosure is devised with the foregoing problems in view, and one of the objects thereof is to provide a control apparatus for an engine, that can suppress the pass-through while achieving a desired engine output. In addition to the above object, advantageous effects that are introduced from each configuration in the following embodiments to carry out the invention but traditional techniques have not achieved, can be also regarded as other objects of the present disclosure.